Are you sure it's been open sourced? I'm reasonably sure you've linked to a site offering pirated copies.

There are several links to PDF versions of the book. None of them include either a copyright page or a statement that it's been released as open source.

The author's own website <https://afu.com/> includes errata for the book, but doesn't provide or mention a free copy.

A free sample of the Kindle version of the book does include a copyright notice. A book published in 1994 is not public domain unless it's been explicitly released.

Something that appears to be a legitimate PDF sample (not the while book) is here:

https://ptgmedia.pearsoncmg.com/images/9780131774292/samplep...